This invention relates to an inverter control system in the D.C. power transmission, and more particularly to a control system which is suitable in case of controlling the gate pulse interval to be constant.
As a phase control system for the gate pulses of an inverter in the D.C. power transmission, the so-called each-phase control system has hitherto been adopted. It determines the phases of the gate pulses of the respective arms of the inverter in such way that gate pulse phase shifters which employ the commutation voltages of the respective arms for their synchronizing power sources are disposed in correspondence with thyristor valves of the arms. In order to perform a stable operation without any commutation failure even when a balance or unbalance fault occurs in an A.C. system on the inverter side in the case of adopting such each-phase control system, control may be made, for example, so as to attain a prescribed extinction angle .gamma..sub.o by detecting a drop of the commutation voltage and increasing the advanced control angle .beta. to the amount of an increment of the overlapping angle ascribable to the drop of the commutation voltage as has been proposed in patent application Ser. No. 78,575/1969.
The control of increasing the advanced control angle .beta. in dependence on the drop of the commutation voltage, however, cannot always achieve the stable operation in case of using a gate pulse phase shifter, as proposed in patent application Ser. No. 129,412/1972, by which phase shifts of the synchronizing power sources are averaged by means of a voltage-controlled oscillator so as to render the gate pulse interval constant. This is obvious when, by way of example, a drop of the phase voltage is supposed. Even if the phase of one commutation voltage leads, that of another commutation voltage will lag to the same amount, so that the averaged phase variation is zero. The foregoing control of increasing the advanced control angle .beta. according to the drop of the commutation voltage cannot cope with the changes of the phases of the commutation voltages. Therefore, the arm at which the phase of the commutation voltage leads falls into an insufficient extinction angle and can cause a commutation failure.